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So many momentous things happened in 1969.

Neil Armstrong walked on the moon, of course, and there were “Easy Rider,” the Stonewall riots, “Abbey Road,” Vietnamization and Arpanet, a precursor to the Internet. The books that year were just as consequential: “Portnoy's Complaint,” “Slaughterhouse-Five” and “The Godfather.”

That was also the year of “The Peter Principle: Why Things Always Go Wrong,” a book by Laurence J. Peter and Raymond Hull that took a humorous look at the way corporations promote employees to their level of incompetence. (“The Peter Principle” was to the business world of that period what “He's Just Not That Into You” was to single women at the beginning of the 21st century.)

And the Peter Principle is probably the single most important cultural artifact hidden in plain sight on “Mad Men,” which returns to AMC on Sunday night for the first half of its seventh and final season.

In the premiere, a condescending client lectures Joan (Christina Hendricks) about the four P's of marketing (price, product, place, promotion). At her agency, though, the Peter Principle thrives: Male ad executives are constantly being promoted beyond their competence level, often over women better suited to the job.

And fittingly, “Mad Men” is living out its own Peter Principle: A series that was so original, fresh and authoritative when it began in 2007 has stayed on television beyond its creative peak.

The season premiere seems as exhausted as the decade it has chronicled so intensely.

The cinematography is striking, as always; the sets and costumes remain as telling as the dialogue – this is when Peter Max was on the cover of Life magazine. But many of the characters are repeating themselves or pedaling in place, and the historic underlay that was once so piquant is now dreary: This season it's the inauguration of President Richard M. Nixon.

That sagging of energy happens to any long-lasting series, but it's oddly apt in the case of “Mad Men,” because the show's trajectory so closely follows the era it portrays.

The year 1969 didn't usher in a new frontier as much as it brought a sour end to seemingly sweet horizons. Woodstock was followed by Altamont; hippie communes were supplanted by the Manson Family; campus peace, love and revolution devolved into the Weather Underground. That was the year that the My Lai massacre came to light, and the Kennedy magic went dark at Chappaquiddick.

Don Draper (Jon Hamm), who at the end of Season 6 was forced by his partners to go on leave after breaking down in the middle of a pitch for Hershey's chocolate, is still in a downward spiral, searching for something even he cannot define. He travels to the West Coast again, but for him, there is no there there, either. And that was true way back in Season 3, when he told a stewardess, “I keep going to a lot of places and ending up somewhere I've already been.”

Roger (John Slattery) keeps running in circles, chasing new variations on sexual promiscuity. Pete (Vincent Kartheiser), so wishfully slick and sophisticated, is as woefully out of step as ever.

It's the women who still have places to go, and that makes sense, because one of the things that actually changed for the better at the very end of the 1960s was the women's movement.

The pleasure of “Mad Men” lies in the clever ways its creator, Matthew Weiner, syncs the milestones of the era, including the Cuban missile crisis, the Kennedy assassination, civil rights, Vietnam and LSD, with the pettier work problems and domestic troubles of Don and his colleagues, wives and lovers.

Women's liberation was never as overtly referred to: Peggy (Elisabeth Moss) was not shown reading “The Feminine Mystique,” and Betty (January Jones) was not immersed in Sue Kaufman's 1967 novel, “Diary of a Mad Housewife.”

Yet all along, feminism has been the moral metronome of the series.

“Mad Men” presented the battle of the sexes as a zero-sum game: Even at the outset, in Season 1, when women in the workplace were secretaries hunting for husbands, not advancement, male primacy was teetering. And that decline was to the quiet benefit of the wives and employees men treated as inferior out of conditioning – and convenience.

Don Draper is an insatiable, relentless womanizer, but his saving grace has been his attitude toward the women he did not seduce: He recognized talent in Peggy and Joan and didn't hold their gender against them.

Now, Joan is still trying to climb the business ladder, even though men keep pulling out the rungs.

Last season, two of the female characters – Peggy and Don's second wife, Megan (Jessica Paré), who seemed well on their way to equality – found themselves at the mercy of men taking steps in their stead.

Megan had a role on a soap opera in New York, when Don chose to move to Los Angeles; he changed his mind, even though she had already quit her job. Peggy thought Ted, her colleague and love interest, would choose her; the next day, he decided to move to Los Angeles with his wife and children.

“Aren't you lucky to have decisions,” Peggy told him bitterly. Her words echo what Betty said to Don in Season 2 when they were married, and he wanted some time alone. “Must be nice,” Betty said coolly, “needing time and just taking it.”

Don watches the movie “Lost Horizon” on late-night television in Sunday's episode, and, as usual, that's a hint. When the decade ends, so will so many of the illusions that sustained it. But one thing has to get better, even on “Mad Men,” and that is the lot of women.